A technology known as “Long-Term Evolution, LIE” has been developed for radio communication in cellular networks. In LTE, different modes of communication can be used for radio nodes in a cellular network such as Frequency Division Duplex (FDD), TDD and half duplex. In this description, the term “radio node” represents any of a base station belonging to a cellular network and a mobile terminal operated by a user.
In TDD, a single physical channel can be utilized for both uplink and downlink transmissions which must be separated in time, in a communication between a base station and a mobile terminal. Therefore, the participating radio nodes are required to switch between transmit mode and receive mode according to a predefined radio frame scheme, thus avoiding that uplink and downlink transmissions occur on that physical channel simultaneously. An example of such a scheme is illustrated in FIG. 1. In this example, a radio frame 100 of 10 ms duration comprises ten sub-frames 0-9 of 1 ms duration each, which can be used for either uplink or downlink transmissions on the same physical channel in a communication. In the Third Generation Partnership Project (3GPP), a number of different uplink-downlink configurations have been defined for sub-frames 0-9 in a radio frame. In FIG. 1, some exemplary arrows are shown within the sub-frames to indicate whether a sub-frame is scheduled for uplink or downlink.
In this figure, an uplink transmission in sub-frame 4 is followed by a downlink transmission in sub-frame 5, implying that the base station must switch from receive mode in sub-frame 4 to transmit mode in sub-frame 5. The mobile station must correspondingly switch from transmit mode in sub-frame 4 to receive mode in sub-frame 5. A single sub-frame 1 is even divided into a field 102 for a downlink Pilot Time Slot, DwPTS, and a field 104 for an uplink Pilot Time Slot, UpPTS, the fields 102 and 104 being separated by a field 106 denoted Guard Period, GP allowing for the above switch and transition of communication modes. This example thus illustrates that both nodes must switch between transmit mode and receive mode in a very accurate and synchronized manner to avoid collisions and disturbances on the physical channel used, particularly between uplink and downlink transmissions.
Different radio nodes, including both base stations and mobile terminals, transmitting in a cellular network are typically required to be mutually synchronized by locking to a common precise reference, such as a pulse emitted from a Global Positioning System (GPS), in order to use a TDD radio frame scheme. It is also common that multiple parallel transmit branches and antennas are employed in a radio node, e.g. to achieve benefits such as diversity, improved data bit rate and/or enhanced signal reception quality, where the same signals are transmitted in parallel over two or more branches and antennas. Some well-known examples of technologies employing parallel transmit branches and antennas are transmit (TX) diversity, Multiple-Input Multiple-Output (MIMO), Beam Forming (BF) and spatial multiplexing. In order to achieve improved performance by using such multiple branches, it is required that the signals emitted from the different antennas are aligned in time, typically also in phase and amplitude.
A simplified example of using multiple transmit branches and antennas in a radio node is schematically illustrated in FIG. 2. The shown radio node 200 may be a base station or a mobile terminal. Any commonly used amplifiers and filters are omitted in this figure for clarity.
The radio node 200 comprises a digital radio part 202 and two branches 204 and 206, denoted A and B, which are used for both transmission and reception of signals through respective antennas 204d and 206d depending on the mode of communication which can be switched as said above. In the digital radio part 202, a signal generator 202a generates signals which are injected to and transmitted over both branches A and B. The signal generator 202a conventionally includes a digital-to-analogue converter, a modulator and an amplifier, which are not shown in this figure for simplicity. The generated signals are first fed to transmit delay buffers 202b and 202c in the radio part 202, which can be pre-configured to delay the signal in time individually in order to calibrate the radio node for output on the two branches and simultaneous emission from the respective antennas 204d, 206d. 
The signals issued from digital radio part 202 are injected to respective transmitters 204a and 206a in the branches A and B, and further pass through a mode switch 204c, 206c in each branch. The mode switch 204c, 206c basically operates to let generated signals pass through towards respective antennas 204d, 206d when in transmit mode, as indicated by full arrows, and to let signals received by antennas 204d, 206d pass through in the opposite direction towards receivers 204b and 206b when in receive mode, as indicated by dashed arrows. The mode switch 204c, 206c is thus controlled to connect the antennas 204d, 206d to either transmitters 204a and 206a or to receivers 204b and 206b, depending on the mode of communication being either transmit or receive mode, in accordance with the prevailing radio frame scheme of sub-frames. The configuration of such mode switches is well known and not necessary to describe in more detail here.
When using such multiple transmit branches and antennas, it is important that the signals are emitted at the same time from the antennas 204d and 206d in transmit mode, otherwise reception of signals on one antenna may be disturbed by transmission of signals from the other antenna, which will be explained in more detail below with reference to FIG. 3 and FIG. 4. Simultaneous emission is also needed to achieve the intended benefit of using parallel antennas. Even though only two antennas are shown in FIG. 2, the above-described arrangement is also applicable for any number of transmit branches and antennas which need to be synchronized in time to avoid misalignment errors.
FIG. 3 comprises a curve 300 that illustrates how output power for transmission from an antenna of a radio node, such as antennas 204d and 206d in the above example, changes over time when switching between receive and transmit modes. First, the output power is at an OFF level when in receive mode. Then at a time t1, transmission is turned on to switch into transmit mode and the output power rises up to an ON level which is reached at a time t2. The period from t1 to t2 is thus a transit period from receive mode to transmit mode. Correspondingly, at a time t3, transmission is turned off to switch back again into receive mode and the output power decreases down to the OFF level which is reached at a time t4. The period from t3 to t4 is thus a transit period from transmit mode to receive mode.
The transit periods t1-t2 and t3-t4 are needed to ramp up and down, respectively, the output power in the radio node according to the shown curve, which can be done during guard periods between uplink and downlink transmissions in the radio frame when no transmission is allowed from either side, such as in the GP 106 shown in FIG. 1. However, if there is a misalignment between two or more parallel transmit branches in a radio node, e.g. the ones shown in FIG. 2, the reception of signals in one branch may be disturbed, or interfered, by a transmission from another branch, thus causing disturbances in the communication. Such a misalignment between transmit branches may also cause severe equipment damages when one branch is still in receive mode and its antenna receives a very strong signal from a closely located antenna of another branch having just switched to transmit mode, or ramping up to transmit mode. The received signal strength in that case exceeds by many times a normal signal strength of signals received from an opposite radio node in normal communication between a base station and a mobile terminal.
This is schematically illustrated by an example in FIG. 4 where a radio node comprises two radio units having two transmit branches each. In this example, transmission from one branch 1B of a first radio unit is delayed in relation to transmission from another branch 1A of the first radio unit, and also in relation to transmission from two branches 2A and 2B of a second radio unit arranged to transmit the same signals, thus causing a misalignment error of At between transmission from branch 1B and transmission from the other branches 1A, 2A and 2B. This misalignment error results in interference from branches 1A, 2A and 2B, while ramping up to the transmit mode during period t1-t2, to branch 1B being still in receive mode during period t1-t2, as indicated by a dashed arrow on the left side in FIG. 4. Correspondingly, branch 1B ramps down from the transmit mode after t4 causing interference to branches 1A, 2A and 2B having already entered receive mode at t4, as indicated by another dashed arrow on the right side in FIG. 4.
It is currently a requirement in 3GPP that the misalignment error between two parallel transmit branches should not exceed a preset limit of 65 nanoseconds to avoid communication disturbances or equipment damages. Therefore, radio nodes are carefully calibrated, e.g. by means of transmit delay buffers coupled to the transmit branches, to fulfill the above requirement. It may still happen that a transmit branch can change its signal propagation time, e.g. due to damage or ageing of components, or malfunction of software, such that the misalignment error exceeds the preset limit which typically goes unnoticed, still resulting in a gradual degradation of performance in the radio node. This performance degradation may involve decreased accuracy in signal detection, decreased data throughput, increased interference, reduction of radio coverage, severe equipment damages, and so forth.